Gas fired appliances, such as residential and light commercial heating furnaces, often use a particular type of gas burner commonly referred to as an inshot burner. In this type of burner, fuel gas under pressure passes through a central port disposed at the inlet of a venturi burner nozzle. Atmospheric air is drawn into the burner nozzle and mixes with the fuel gas as it passes through the burner nozzle. In some constructions, a burner head insert, also referred to as a flame retention insert, made of compressed sintered or powdered metal is mounted in the outlet end of the tube. In operation, gas is injected into the inlet end of the nozzle, entraining air into the nozzle with it. This primary air/gas mix flows through the tube to the burner head insert. The primary air/gas mix passes through outlet openings defined by the insert and burns as it exits the insert forming a flame projecting downstream from the burner head insert. Secondary air flows around the outside of the venturi tube and is entrained in the burning mixture downstream of the insert in order to provide additional air to complete combustion.
Some of the problems associated with conventional inshot burner designs are ignition, flame stability, noise and nitrogen oxides formation. Ignition problems can arise when the ignition location is critical. The velocity of the primary air/gas flow from the insert is often greater than the flame speed. Under this condition, the flame lifts off from the burner insert, i.e. the flame begins to burn at a location spaced from the outer face of the flame retention insert. Flame liftoff is a major cause of the noise associated with the operation of inshot burner nozzles. If the velocity of the air/gas mixture is too slow when compared to the flame speed, flashback can occur. Flashback is the burning of the gas within the burner nozzle itself. This condition can cause overheating and deterioration of the nozzle.
Various flame retention or burner head inserts have been designed in the past in an attempt to achieve better flame stability and reduction of noise. One known insert comprises a cylindrical body defining a central opening and having a toothed perimeter formed by a plurality of circumferentially spaced, axially elongated splines extending radially outwardly in a sunburst pattern about the circumference of the cylindrical body. In conventional prior art sunburst type inserts, each spline comprises an axially elongated bar of rectangular cross-section. The air/gas mixture passing through the central opening of the insert forms an inner flame cone, while the air/gas mixture passing through the circumferentially spaced openings between the sunburst arrayed splines forms an outer flame cone. Another known insert has a central opening surrounded by a series of circumferentially spaced, small holes. In both designs, the velocities of air/gas flow through the outer series of openings and the central opening are nearly the same.
During the combustion of fossil fuels, including gaseous fuels such as natural gas, liquefied natural gas and propane, for example, in air, oxides of nitrogen (NO.sub.x) are formed and emitted to the atmosphere in the combustion products. With respect to gaseous fuels which contain little or no nitrogen per se, NO.sub.x is formed as a consequence of oxygen and nitrogen in the air reacting at the high temperatures resulting from the combustion of the fuel. As oxides of nitrogen emitted to the atmosphere are considered a noxious pollutant, it is desirable to reduce the formation of nitrogen oxides within the flame.